Class 11 Chemistry Chapter 11 The p-Block Elements
NCERT Notes For Class 11 Chemistry Chapter 11 The p-Block Elements, (Chemistry) exam are Students are taught thru NCERT books in some of state board and CBSE Schools. As the chapter involves an end, there is an exercise provided to assist students prepare for evaluation. Students need to clear up those exercises very well because the questions withinside the very last asked from those.
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NCERT Notes For Class 11 Chemistry Chapter 11 The p-Block Elements
Class 11 Chemistry Chapter 11 The p-Block Elements
THE p-BLOCK ELEMENTS
These are elements in which the last electron enters in the outer most p-subshell. They include elements of groups 13 to 18.
Their general outer electronic configuration is ns2np1-6 (except for He). They include metals, non-metals and metalloids.
Their maximum oxidation state = the total no. of valence electrons (i.e., the sum of the s- and pelectrons). But their common oxidation state may differ from the maximum oxidation state or group oxidation state.
Generally, in the p-block elements, down the group, the common oxidation state is 2 less than the maximum oxidation state.
This is due to the Inert pair effect.
It is the reluctance of s-electrons to participate in chemical bonding.
It is commonly seen in the elements of groups 13, 14 & 15. [Down the group, due to the poor shielding effect of inner d and f orbitals, the effective nuclear charge is greater, which holds the s-electrons tightly. So they cannot participate in bonding].
Due to the above reason, TlCl is more stable than TlCl3. Similarly lead mainly form PbCl2 than PbCl4.
Or, Tl+ is stabler than Tl3+ and Pb2+ is stabler than Pb4+).
Group 13 Elements (Boron Family)
Group 13 include Boron (B(, Aluminium (Al), Gallium (Ga), Indium (In), Thalium (Tl) and Nihonium (Nh).
Among these elements, Boron is a typical non-metal and the other elements are metals.
Atomic radii: Down the group, atomic radius increases.
But atomic radius of gallium is less than that of aluminium.
This is due to the presence of completely filled d-orbitals in Ga (Ga – [Ar] 3d10 4s2 4p1).
The presence of 10 d-electrons offer only poor shielding effect for the outer electrons, from the increased nuclear charge.
Ionisation Enthalpy: The ionisation enthalpy values do not decrease smoothly down the group.
This is due to the poor shielding effect of the completely filled inner d and f electrons.
Oxidation state: The common oxidation state of 13th group elements is +3.
Due to high ionisation enthalpy, boron does not form +3 ions and it only forms covalent compounds.
The stability of +3 oxidation state decreases and that of +1 oxidation state increases down the group.
This is due to inert pair effect.
The trivalent compounds formed by 13th group elements are called electron deficient compounds.
In these compounds, the number of electrons around the central atom of the molecule is only 6.
In order to attain stable octet configuration, they accept a pair of electrons and so they behave as Lewis acids. e.g. BF3, BCl3, AlCl3, B2H6 etc.
Chemical properties
Reactivity towards air: Boron is unreactive in crystalline form.
Due to the presence of an oxide layer on the surface, Al does not react with air at normal temperature. But at high temperatures, they form oxide and nitride.
2E + 3O2 → 2 E2O3
2E + N2 → 2 EN [E = Any 13th group element]
The oxide of boron (B2O3) is acidic, the oxides of Aluminium and Gallium are amphoteric and that of Indium and Thalium are basic.
Reactivity towards acids and alkalies: Boron does not react with acids and alkalies even at moderate temperatures.
But aluminium dissolves in mineral acids and aqueous alkalies and thus shows amphoteric nature.
But it does not react with nitric acid due to the presence of the oxide layer on the surface.
Reactivity towards halogens: They react with halogens and form trihalides.
2E(s) + 3X2(g) → 2EX3(s)
2Al(s) + 3Cl2(g) → 2AlCl3(s)
AlCl3 exists as dimer to attain stability.
Anhydrous aluminium chloride is partially hydrolysed with moisture to liberate fumes of HCl gas. So white fumes appears around the bottle of anhydrous AlCl3.
Some important compounds of Boron
1. Borax [Na2B4O7.10H2O]: It is a white crystalline solid with formula Na2B4O7⋅10H2O (Sodium tetraborate decahydrate). It contains the tetranuclear units [B4O5(OH)4]2- and hence its correct formula is
Na2[B4O5 (OH)4].8H2O.
It dissolves in water to give NaOH and orthoboric acid. Since NaOH is a strong alkali and orthoboric acid is weak acid, the solution is basic in nature.